Its Out of America

Woolly mammoth skeleton at the American Museum of Natural History.

Credit: Denis Finnin, AMNH

In an unexpected twist, new research shows that the last of the woolly mammoth--seven those whose fossils were found in Asia--originated from a single population. Genetically, they all were North American. This result, dubbed the 'Out of America' hypothesis, is contrary to previous thinking and based on a new analysis of ancient DNA collected by Ross MacPhee, Curator of Mammalogy at the American Museum of Natural History, and his colleagues.

"North America was thought to be a side show," says MacPhee. "But the genetic research shows that woolly mammoths of North America were actually the source of the entire population that lived throughout the Arctic until their final extinction several thousand years ago. This challenges traditional paleontological ideas."

Woolly mammoths diverged from more primitive mammoths in Siberia less than a million years ago and moved into North America via a land bridge about 200,000 years ago. Because there were few migratory events over the land bridge since then, the two populations remained relatively isolated from each other and became genetically distinctive, although this was not known until recent advances in technology allowed geneticists to analyze degraded or 'ancient' DNA in fossils. Much of the previous research was based on morphology, and because bones and teeth do not reveal complex population changes at the molecular level, scientists assumed that all seminal events in the evolution of Mammuthus primigenius occurred in the Old World.

The new genetic research, published in Current Biology this September, upends this assumption. Taking samples from 108 specimens (in addition to previously published DNA sequences of another 52 individuals), MacPhee and colleagues from the McMaster Ancient DNA Centre at McMaster University in Ontario found that all woolly mammoths that lived from approximately 40,000 years ago to their final extinction resembled the North American population at a molecular level, not the older population of Siberian mammoths.

"Small-scale replacements of populations are not a rare phenomenon within species, but replacements on a continental scale are rare," says MacPhee. "But although the genetic evidence gives us insight into population movements within the species, it does not tell us why the North American group replaced the Asian."

There are two possibilities: first, the Eurasian mammoth population was already in decline, and the movement of North American groups into Eurasia was coincidental; alternatively, the North American population out-competed the older Eurasian group. In either case, all mammoths went extinct in the Late Quaternary, along with a number of other large mammals such as saber tooth cats, ground sloths, and mastodons. These changes began immediately after the arrival of humans and are probably related in part to human activities like hunting, but other root causes are also possible, such as disease, climate change, or perhaps a small meteor or asteroid impact 12,000 years ago (for more information, see www.amnh.org/sciencebulletins/biobulletin/biobulletin/story981.html).

Other researchers that contributed to this paper include, among others, geneticists Hendrik Poiner and Regis Debruyne of McMaster University and many individuals who contributed specimens for analysis. These individuals and MacPhee received grants from the National Science Foundation, the Niarchos Foundation, the Natural Science and Engineering Research Council, the Canadian Research Chairs program, and a Quest grant from Discovery Communications.